A driver cannot see a pedestrian standing just around a sharp $90^\circ$ corner of a high brick wall unless…
A driver cannot see a pedestrian standing just around a sharp $90^\circ$ corner of a high brick wall unless a mirror is installed at the turn. Which fundamental property of light prevents the driver from seeing around the corner directly?
Light bends around solid corners naturally.
Light travels in straight lines and cannot curve on its own around obstacles.
Light gets completely absorbed by air at right-angle turns.
Light slows down when passing close to brick walls.
Solution
Light travels along straight lines (rectilinear propagation), so it cannot bend on its own around a physical obstacle to reach the driver's eyes without a reflecting surface.